1. Field of the Invention
This invention relates to a method of and an apparatus for reproducing information by irradiating an information carrying member with light and by detecting the light modulated by the information recorded on the information carrying member.
2. Description of the Prior Art
In general, an optical information reproducing apparatus includes a light source such as a laser or the like, optical systems such as a collimator lens, objective lens or the like, and a light receiving element such as a pin-photodiode or the like. In the reproducing apparatus as such, the light beam emerging from the light source is converged on an information carrying member so that the light is optically modulated in accordance with the information recorded on the information carrying member, whereafter the modulated light beam is photoelectrically converted by the light receiving element, thus converting the recorded information into electrical signals. Such optical information reproducing apparatus is applicable to an optical video disc, optical digital audio disc, optical information file or the like. Usually, the recorded information comprises fine patterns formed concentrically or spirally on a rotatable disk-like information carrying member (disc). These fine patterns are defined by surface unevenness, partially formed holes, variation in the reflection factor, variation in the direction of magnetization or the like. In any application of the reproducing apparatus, the patterns must be minute in order to improve the recording density. For modulating the light beam with such minute patterns, the minimum spot diameter of the beam converging on the information carrying member is determined by the numerical aperture (NA) of the lens used in the optical head, the wavelength of the light to be used and the like, and has a finite value.
On the other hand, it is considered that the converged beam spot has, in its cross-section orthogonal to the optical axis, a light distribution which is usually similar to the Gaussian distribution. And in the modulation of such light beam by the fine patterns, the degree of modulation by those by fine patterns as having high spatial frequencies is lowered owing to the modulation transfer function (MTF) so that the noise margin of the overall system is diminished, as will be seen from FIG. 1 of the accompanying drawings which shows the spatial frequency transmitting characteristic in a case of using a single beam spot.
Where the converged beam traces the minute pattern array on the recording disc at a constant linear speed, the amplitude of the reproduction signal depends on the frequency of the reproduction signal (i.e., a higher frequency leads to a smaller amplitude). On the other hand, in a case of tracing with a constant angular speed, the aforesaid frequency-amplitude characteristic changes with the variation in the linear speed depending on the tracing position of the converged beam spot.
As the techniques for supressing the effects of the modulation transfer function in such type of optical head, there are known:
.circle.1 line equalizing method by a high-pass filter using a LCR filter and an active filter, and by a band-pass filter; and
.circle.2 line equalizing method by a transversal filter using a delay element.
By the modulation transfer function of a lens system, only the frequency-amplitude characteristic of the signal is affected while the frequencyphase characteristic is not affected. However, according to the above-noted method .circle.1 , it is difficult to change only the frequency-amplitude characteristic without changing the frequency-phase characteristic. Therefore, even if the diminution of the amplitude in the great range owing to the MTF can be cancelled, there would still occur phase distortion.
Further, the above-noted methods .circle.1 and .circle.2 are both the signal processing methods with respect to the time region (i.e., they are not signal processing methods with respect to the spatial region). Accordingly, while they are applicable to such a system in which "time" and "space" correspond to each other, e.g. such a system in which the recording disc is rotated so as to maintain a constant tracing speed of the recording pattern to reproduce the information signal, they are not applicable to such a system in which the recording disc is rotated at a constant angular speed since in this case the frequency characteristic of the filter means must be changed in accordance with the tracing position of the converged beam spot. Furthermore, the above-noted method &gt;.circle.2 is not advantageous in that it requires a delay element which leads to increase in the cost.